In spite of improved treatments for certain forms of cancer, it is still a leading cause of death in the United States. This is true not only for humans, but also for domestic and companion animals such as dogs, cats, horses, and cattle. For example, millions of dollars are lost each year due to the forced slaughter of cattle that have lymphosarcoma. As another example, lymphosarcoma is the most common hematopoietic malignancy of dogs, with an incidence of about 0.1-0.3% in the general population. Without chemotherapy, canine lymphosarcoma runs a rapid course and is uniformly fatal.
In cases where the primary tumor in an animal has been substantially removed by surgery or destroyed by other means, it is important that the veterinarian or pathologist be capable of detecting any trace of cancer in the animal (either in the form of residues of the primary tumor or of secondary tumors caused by metastasis), in order that the veterinarian can prescribe appropriate subsequent treatment, such as chemotherapy. The quantities of cancer cells that must be detected for early diagnosis or following removal or destruction of the primary tumor are so small that the veterinarian or pathologist cannot rely upon physical examination of the cancer site. Moreover, in many cases the cancer site is of course not susceptible to direct visual observation, and it is not possible to predict exactly where cancer is likely to occur. In addition, even when tissue indicative of a malignancy can be visually detected, histologic differentiation of the malignancy can be difficult, even for experienced pathologists. Accordingly, sensitive tests have to rely upon detection of cancer-associated materials, usually proteins, present in body fluids of animals who have, or are about to develop, cancer cells in their bodies.
Several diagnostic materials for detection of cancer-associated proteins are available commercially. Tests for alpha-fetoprotein are used to detect primary liver cancer and teratocarcinoma in humans; carcinoembryonic antigen is used for digestive system cancers, as well as lung and breast carcinomas; chorionic gonadotropin is employed to detect trophoblast and germ cell cancers; calcitonin is used for thyroid gland cancers; and prostatic acid phosphatase is used to detect prostate carcinoma. These markers are detectable in advanced rather than in early cancer.
Unfortunately, many of the commercially available tests are only applicable to a narrow range of cancer types, and therefore these tests suffer not only from the disadvantage that other types of cancer may be missed but also from the disadvantage that the narrow applicability of the tests means that it may be necessary to run multiple tests on a single patient or animal for diagnostic purposes, a procedure which not only increases the expense of the diagnostic testing but also increases the risk that one or more of the tests may give a false positive result. Accordingly, there is a need for a single diagnostic test able to detect the presence of very small amounts of cells of a wide variety of different cancers. The ideal marker would be one that is specific and universal. Such a marker may exist if malignant transformation is associated with the expression of a unique gene product in all kinds of transformed cells.
It is already known that serum from the blood of animals suffering from a wide variety of cancers contains an oncofetal protein having a molecular weight of approximately 60,000 and having the capacity to increase the release of ribonucleic acid (RNA) from cell nuclei. This protein is referred to as oncofetal RNA-transport protein (ORTP) or 60 kDa tumor-associated protein.
ORTP is localized in the cytoplasm of tumors of humans and experimental animals and small amounts are released into the host circulatory system. The 60 kDa ORTP is notably absent from the nuclei of rat liver and rat liver tumors. It has been shown to be present in fetal rats at 18 days of gestation and in human and rat amniotic fluid, but not in maternal blood. It has not been detected in adult rats. Nor is it present in detectable concentrations in the blood of normal human subjects or those with a variety of non-neoplastic conditions or diseases, including benign tumors and other non-neoplastic proliferative diseases. In contrast, of more than 200 cancer patients with confirmed active disease, all tested positive for the factor. It was also present in all of about 200 tumor-bearing rats tested.
Unfortunately, antibodies to the rat 60 kDa cancer-associated protein preparation purified as described in the prior art do not cross-react with human or mouse ORTP. Thus, the 60 kDa cancer marker protein from different species are not immunologically equivalent, e.g., an antibody to the rat cancer marker protein does not cross-react with a human or mouse cancer marker protein. Thus, when the purified 60 kDa cancer marker protein preparation is to be used for production of antibodies for diagnostic purposes, it is probably necessary to begin the preparation process with plasma from the species in which the diagnosis is to be used.
Previous attempts to use RNA-transport-stimulating oncofetal proteins as markers to monitor the long-term effects of cancer treatment have not been successful. In particular, there has been a need for improved methods of monitoring the response of animals to treatment for cancer, in order to verify remission after treatment and to detect the onset of a recurrence of cancer.
We have now invented an improved method for monitoring the status of cancer treatment in animals, an improved adjunctive diagnostic test for malignancy in companion and domestic animals, and an improved method of preparing monospecific antibodies for use in the monitoring method.